1. Field of the Invention
The present invention relates to a fault-sensing and protecting apparatus for soft start circuit, especially to a fault-sensing and protecting apparatus for soft start circuit used in an inverter.
2. Description of Prior Art
AC motor has been widely used as the mature of solid state inverter. The AC motor has the advantages of small rotor inertia, and excellent heat dissipation effect, and is extensively used in CNC machine and industrial robot as the progress of power semiconductor, microprocessor and control industry.
FIG. 1 shows a block diagram for a prior art AC motor connected to a wall socket power. An AC power such as 110Vac or 220Vac is supplied from a wall socket power 20 to a converter 30 for AC to DC conversion. The converted DC power is then sent to an inverter 40 for DC to AC inversion. The inverter 40 is controlled by a central processing unit (CPU) to obtain an AC power with required frequency and level. The AC power with required frequency and level is used to drive a motor 60.
The inverter uses a rectified and filtered DC power as input and therefore a soft start circuit with resistor is employed in the inverter for suppressing starting current. After starting, electrically-controllable switch unit such as relay, electromagnetic contact or SCR switch is used for bypassing the soft-start resistor. As shown in FIG. 2, the converter 30 is mainly composed of diode set 32 and filtering capacitor C. The wall socket power 20 is supplied to the inverter (not shown) after being rectified by the diode set 32 and filtered by the filtering capacitor C. When the voltage different between the filtering capacitor C and the wall socket power 20 is large, a charging current is considerable. A large starting current is generate to damage the diode set 32, burn out fuse or turn off a fuse-less switch. Therefore, the soft-start resistor is added to suppress the starting current. More particularly, a soft-start resistor R is provided in parallel with the rectifying and filtering circuit. The soft-start resistor R consumes considerable power when the circuit starts. Therefore, switch unit such as relay, electromagnetic contact or SCR switch is used for bypassing the soft start resistor. The switch unit and the soft-start resistor R are referred to as soft-start circuit. As shown in FIG. 2, the CPU controls the relay 72 to close the switch end 72A thereof (normally open end) for bypassing the soft start resistor R after the inverter starts.
However, the soft-start circuit malfunctions when printed circuit board (PCB) has line breaking problem, or the relay breakdowns. There is no protection circuit or malfunction detecting circuit for the soft-start circuit, and therefore the inverter has burning down risk after the soft-start circuit malfunctions. The soft-start resistor R has risk of burning down if the load is large (the load current is large). This happens when the soft-start resistor R is not bypassed. It is desirable to provide a protection circuit and malfunction detecting circuit for the soft-start circuit.